1. Field of the Invention
This invention relates to an improved electrochromic display device comprising, in a cell, a display electrode a counter-electrode and an electrolyte containing organic electrochromic material.
2. Prior Art
Electrochromic display devices are well known by, for instance, an article by C. J. Schoot et al in Applied Physics Letter, vol. 23, No. 2 published July 15, 1973. In the abovementioned article, N,N'-di-n-heptyl-4,4'-dipyridinium dibromide is used for the electrochromic display substance and pottasium bromide is used for the supporting electrolyte. These materials are disolved in water to make solutions having the concentrations of 0.1 mol/l and 0.3 mol/l in the electrolyte, respectively. A display electrode and a counter-electrode of chemically inert material are disposed in the electrolyte. Furthermore, a reference electrode made with silver and silver bromide is disposed in the electrolyte. In such conventional electrochromic device, the reversible electrochemical reaction of the belowmentioned chemical formula (1) takes place on the display electrode upon writing and erasing: ##STR1## wherein: R and R' are groups are selected from alkyl groups, phenyl groups, alcoxycarbonylalkyl groups, phenylalkyl groups, and the like.
X.sup.- is an electrochemically inactive monovalent anion selected from Br.sup.-, BF.sub.4.sup.-, ClO.sub.4.sup.-, and the like.
At first, the 4,4'-dipyridinium compound (I) of the abovementioned formula is disolving in the electrolyte in an almost transparent state. Then by applying a specified voltage for producing an electrochemical reduction reaction of the compound (I), the compund (I) is electrochemically reduced to form monocation radical (II) of the 4,4'-dipiridinium compound which radical (II) displays a purple color.
The problem of the conventional electrochromic device is that the monocation radical (II) is further reduced by a lower negative potential to form a diradical (III) shown in the electrochemical formula (2) below: ##STR2## The formation of diradical proceeds irreversibly in the reaction (2) and therefore, the reaction (2) makes erasing of the writing of an image difficult.
Accordingly, in the conventional electrochromic display device, it is necessary to conduct the electrochemical reaction within a potential in a reversible reaction region by carefully controlling the potential of the display electrode. Thus, it has been necessary to provide a reference electrode as described in the abovementioned article.
However, even in such a device of the abovementioned article, it has been difficult to achieve an entirely reversible electrochemical reaction in the device. The difficulty is caused by an irreversibility of the belowmentioned reaction (3) on the counter-electrode: EQU 2X.sup.- -2e.revreaction.X.sub.2 ( 3)
For example, when bromide ions are used for the elctrochemically inactive anions X.sup.-, an oxidation reaction takes place on the counter-electrode at a electrode potential within a region more positive than about +0.65 V. Furthermore, in this case, a yellow deposit believed to be a bromine complex of the 4,4'-dipyridinium compound is formed on the counter-electrode. Since the electrochemical reduction rate of this oxidation product is very slow, the rate of the abovementioned oxidation reaction becomes the determining rate of the displaying reaction.
Besides the abovementioned reaction to form the complex compound, another reaction according to the formula (1) takes place before initiation of reaction of the formula (3). Accordingly, a further complicated irreversible electrochemical reaction is made on the counter-electrode.
Additionally, since a reference electrode is used, the operating circuit of the electrochromic device of the abovementioned article is complicated.
FIG. 1a shows potential-current curve on the display electrode and FIG. 1b shows the similar characteristic on the counter-electrode. Both diagrams show electrochemical behavior of N,N'-di-n-heptyl-4,4'-dipyridinium dibromide vs. a saturated calomel electrode (SCE). In these diagrams, in the region A, which is a region more negative than a specified negative potential En of about -0.4 V, a reduction reaction of the electrochromic material takes place thereby making a reduction current and a writing. When the potential becomes higher than En and is between the potential En and a specified positive potential Ep of about +0.65 V, the electrochromic material reduced in the abovementioned reduction reaction in the region A is oxidized, thereby making an oxidation current and an erasing. When the potential is in the C region, namely higher than Ep, anions are oxidized thereby making an oxidation current. When the operating point again comes into the low potential region A, the abovementioned oxidation product of anions is reduced making a reducing current. Accordingly, the erasing of the display is possible.
If the abovementioned reactions on the display electrode are completely reversible to the reactions on the counter-electrode, then the electrochromic device performs ideally without any deterioration. However, as already mentioned, the redox reaction on the counter-electrode is far from an ideal reversible type.
We have made a substantial advance in the art by providing electrochromic display devices having improved performance characteristics as more fully described below.